I'll answer you with my example.
Registered in July 2017.
Appointed mod of the french section in July 2018 with the rank of member and 13 or 14 merits.
I wasn't the most active. I am far from having all the knowledge about Bitcoin. I'm not the most fluent in english, I hope I'm just understandable.
So you see the criteria for a local mod are not high. It must be higher for a mod in US section or a Global Mod.

As quoted above, the number of reports is the most important criteria. It's a secret for nobody. But maybe Theymos has some secret criteria (number of deleted messages, your "Total time logged in" to see if you are enough present, no red trust, etc...) ?
And I would add that you don't have to be a member too controversial. Imagine that [well... I will not give a name, but you all have at least one in mind] is appointed moderator of a section, and it will be chaos and desolation on Bitcointalk